The present invention relates to a flexible tubular pipe of the unbonded type, intended especially for the offshore oil industry, for example for transporting hydrocarbons, and produced by independent successive layers consisting, on the one hand, of helical windings of various profiled strips and/or tapes, especially made of metal, and, on the other hand, of at least one sheath made of polymer material. The various successive layers have a certain degree of freedom in moving with respect to one another, thereby ensuring good flexibility of the pipe.
A standard type of such flexible oil pipes comprises, from the inside outward: an internal carcass consisting of a short-pitch helical winding of a profiled strip (generally a metal strip) in mutually interlocked turns; a polymeric internal sealing sheath, or pressure sheath, pressing on the carcass and thus able to withstand external pressures without the risk of being crushed; a set of armor plies consisting of helical windings of profiled strips and intended to withstand in particular the hoop stress and the axial component of the internal pressure, and also the axial load due to the weight of the suspended pipe; and a polymeric external protective sheath. The set of armor plies is usually divided into two subsets more particularly responsible for taking up the radial stresses and the axial stresses respectively, namely the pressure vault and the tensile armor plies: the pressure vault is placed over the internal sealing sheath and is intended to withstand the external pressure but mainly the internal pressure developed by the fluid in the sealing sheath, and it generally comprises a short-pitch helical winding of an interlocked profiled wire (that is to say a winding with a wind angle of typically between 75° and about 90° to the axis of the pipe); the tensile armor plies are generally noninterlocked wires wound helically with a long pitch (i.e. with a lay angle of less than 55°) in at least two crossed plies on top of the pressure vault. However, in certain cases it is possible to have a set of armor plies consisting of only 55°-wound cross armor plies with no pressure vault.
Such a pipe, in its general form, is well known to those skilled in the art, especially from the standardized documents “Recommended practice for flexible pipe 17B” and “Specification for Unbonded Flexible Pipe 17J” from the American Petroleum Institute.
It has been known for a long time to use, in such a pipe, windings of very long tapes made of various extrudable polymers. These windings are interposed between various layers of metal windings and/or sheaths of the pipe with a great variety of technical functions.
There may for example be anti-wear or rub tapes, provided by the abovementioned document API 17J and illustrated for example in documents U.S. Pat. No. 5,730,188 or WO 01/33129, which tapes may be placed under, between and/or over the various armor plies.
There may be tapes for mechanical confinement and thermal and/or chemical protection, especially interposed between the carcass and the sealing sheath.
This is because the internal sealing sheath is commonly made either of a polyamide, especially nylon-11 (PA-11), such as RILSAN B®, or made of polyvinylidene fluoride (PVDF). In the first case, the operating temperature is limited as PA-11 has a susceptibility to hydrolysis that imposes a temperature limit in the presence of water, which depends on the planned lifetime of the pipe and on its design, but which, in certain cases, is about 90° C. In the second case, PVDF is limited in terms of service temperature, to about 130° C. or even 120° C., for other reasons (change in crystallinity and susceptibility to crack propagation after aging). Now, temperatures above 130° C. are encountered in the production of live crude. This is why manufacturers are proposing the interposition of a barrier layer between the carcass and the internal sealing sheath.
Thus, documents EP 0 749 546 and EP 0 817 935 teach a flexible pipe of the aforementioned type which includes, in addition, between the internal carcass and the internal sealing sheath, a sublayer made of elastomer, this sublayer being, in the most general embodiment, usually produced by extrusion. However, also provided, as a variant, are embodiments in which the sublayer is formed by an elastomer tape wound helically along the interstices or gaps in the internal carcass and penetrating to a greater or lesser depth into the interstices. This tape may also be made of a relatively soft elastomer and wound with touching turns in order to cover the entire internal carcass and to be partly forced into its gaps. The thickness of such an additional internal layer is around 0.5 mm to 5 mm (preferably 3 mm), whereas the polymeric sheath has a thickness of 1 to 30 mm (preferably 3 to 15 mm) for a metal carcass diameter of between 20 and 600 mm (preferably between 50 and 400 mm), said additional internal layer being designed to withstand internal pressures of greater than 100 bar, possibly pressures of up to or exceeding 1000 bar, and also to withstand high temperatures, exceeding 130° C. or even 150° C. The aforementioned documents do not give details about the preferred elastomers for the embodiment of these helical tapes, which are only one particular embodiment of a sublayer of the internal sealing sheath. In general, the elastomers used are proper elastomers, normally in the vulcanized or crosslinked state, or thermoplastic elastomers (TPES), these being chosen so that some of their properties are not degraded by the combination of the action of the various components present in the transported fluid and of the temperature of this fluid, while the material exposed to such conditions ages. Among the very many elastomers mentioned, thermoplastic polyolefin elastomers (TPOs) and fluoroelastomers are in particular noted. It has also been indicated that useful results are obtained with elastomers belonging to the silicone group. Document WO 02/090818 describes a flexible pipe of the same type with an intermediate layer wound between the carcass and the internal sealing sheath. The winding tapes may form a layer 5 to 10 mm in thickness. Examples of materials used are in particular polymers comprising 50% by volume of polyolefins (TPOs), as in the first two documents mentioned, polyketones, or other materials such as XPEs (crosslinked polyethylenes), PVDF and polypropylene.
According to document WO 02/066878 in the name of the Applicant, various possible constituents for the composition of a tape are given: one passage indicates that the tapes must be made of polyolefins, polyamides (preferably of the RILSAN® nylon-11 type), fluoropolymers (homopolymers or copolymers), whether modified or not (polyvinylidene fluoride (PVDF) or polyfluoroalkoxy) or hydrocarbon elastomers, fluoroelastomers or fluorosilicone elastomers (thermoplastic elastomer or thermoplastic urethane). This is therefore a very long list of materials having very varied properties, but these are obviously not suitable for the problem posed by the present invention since the limitations of some of the materials recommended in that document, such as RILSAN® or PVDF, have already been explained above.
Although many materials have thus been proposed, the ideal material, combining the possibility of being wound in the form of helical tapes and effective mechanical and chemical and thermal barrier properties, remains to be found.